Process for drying sewage sludge or sludges of similar consistency

ABSTRACT

In a process for drying sewage sludge or sludges of similar consistency, use is made of a device (13) in which vapors arising during contact drying are introduced to at least one compression stage. This stage compresses the vapors to a higher pressure and impurities are separated therefrom. The cleaned and compressed vapors are introduced into the device (13) for contact drying as a heating medium. In this manner, the energy consumed inside the operated installation is used more efficiently to dry the sewage sludge and a considerably improved energy balance is thereby achieved.

BACKGROUND OF THE INVENTION

The invention concerns a process for drying sewage sludges or sludges ofsimilar consistency with which impurities are separated from vaporsoccurring during drying.

A method of this kind has become known in the art through DE 39 43 366A1.

Environmental protection has been a central theme in public discussionsin the industrialized nations during the last two decades. As a resultthereof, a plurality of legal requirements have been created e.g. in theEuropean Economic Community. Corresponding action has been taken by thestate and community authorities and by the industrial sector as a resultof which an improved sewage treatment, i.e. sewage processing orpurification, has been achieved.

Beginning with the 1976 guidelines of the EEC concerning pollution dueto drainage of certain dangerous materials into the waters of the EECand up to the EEC Guide Lines of May 1991 concerning the treatment ofcommunity sewage and improvements in the general treatment of sewage,there has been a yearly increase in the overall amount of effortexpended and the associated funding in order to effect improvedpurification of sewage from industrial installations and privatehouseholds. As a logical consequence of sewage purification, recentyears have seen an increase in the associated amount of sewage sludge.This will increase even further in the coming years. One has thereforesearched for a solution to the problem of how this sewage sludge, anunavoidable waste product of sewage treatment, can be further processed,utilized or disposed of. Sludges having similar consistency occur inindustrial installations e.g. in painting installations, washinginstallations of all kinds and the like.

Since the disposal of sewage sludge is becoming increasingly difficultand expensive due to the shortage of available dumping facilities, amass and volume reduction in the amount of generated sewage sludge isnecessary prior to disposal or further processing. The dumping of sewagesludge will be forbidden by law beginning in the year 2005. A reductionof volume simplifies transportation problems as well as the handling ofthe sewage sludge. This concerns both the directed treatment e.g.purification, further processing and, if appropriate, even the recyclingof the sewage sludge.

Various methods are utilized either separately or collectively to reducethe mass and volume of the sewage sludge involving, for example,influencing evaporation of the moisture bound within the solid sludgecomponents (drying), the decomposition of organic materials throughremoval of gases as well as the oxidation of organic substances throughincineration.

DE 39 43 366 A1 proposes processing steps relating to the temperature ofa fluidized bed, to the introduction of solids into the fluidized bedwhich have large amounts of material which can evaporate, to the removalof dried solids from the fluidized bed, as well as to the gaseouscomponents of the evaporating material removed along with the vapor andother gaseous impurities.

An energy balancing of these processes of treatment and utilization ofthe sewage sludge takes into consideration the fact that energy must beexpended in order to carry out the mechanical liquid removal processesand the thermal drying including processing of the vapours. In contrastthereto, energy is freed when the dried sewage sludge is subsequentlyincinerated in a power plant. If the sewage sludge is dried, andsubsequently incinerated without prior decomposition thereof, the energybalance is compromised. However, if a decomposition process is carriedout, a longer processing time disadvantageously results, since thedecomposition processes require a certain amount of time. Aself-sufficient operation with regard to energy is not possible, sinceit is always necessary to use more energy than is freed. In order toinfluence the energy balance during subsequent treatment in a positive,environmental manner, measures for improvement of the drying process arerequired.

It is therefore the purpose of the present invention to optimize theconventional thermal drying process in such a fashion that the energyutilized during operation of the installation is more efficientlyapplied for drying the sewage sludge to achieve as positive an energybalance as possible subsequent to incineration of the dried sewagesludge.

This purpose is achieved in accordance with the invention with a methodfor drying sewage sludge or sludges of similar consistency in whichsewage sludge or sludges of similar consistency are processed in acontact drying device from which the vapors occurring during contactdrying are introduced to at least one compressor in which the vapors arebrought to a higher pressure and impurities are separated out of thevapors, and the purified and increased pressure vapors are utilized as aheating medium in the contact drying device.

A contact drying device is used for effective heating of the sewagesludge.

SUMMARY OF THE INVENTION

Up to this point in time e.g. a bag filter was required for separationof liquid and solid vapor components and a condensating device wasneeded to condense the vapors. The resulting cooling water had to beremoved and the residual vapors introduced to an additional preparationsystem e.g. a filter for deodorization. Energy was required for eachindividual step.

One can advantageously avoid the above mentioned bag filters utilizedfor drying of the sewage sludge up to this point in time or replace themwith one single device having a condensating step i.e. with a compressorin which the vapors can be compressed. A plurality of compression stagescould also be used within the device.

Vapors are brought to an increased pressure of preferentially 1 to 3 barand thereby to a higher condensation temperature in the compressor whenthe required compression pressure is sufficiently high that the boilingpoint increase is surpassed and a sufficiently large temperaturegradient is available for heat transfer. The important operationparameters for choice of a suitable compressor are the pressure increaseand the volume flow of the vapor to be compressed. When these conditionsare given, vapors can be separated into solid components, substantiallyorganic solids, and vapors, primarily water vapor. The solid organiccomponents can be separately removed. The hot additionally compressedwater vapor is, in contrast thereto, no longer fed to the condensatingdevice and cooled rather introduced directly as a heating medium to thedevice for contact drying. The vapors could be solvent-containing vaporsand the separation of the vapor into solid components and gases can takeplace directly in the compressor.

The furnace which has been used up to this point in time to produce theheating medium can therefore advantageously be more compact or only beutilized to initiate the drying process.

The energy needed to operate the condensing device as well as energyrequired for operation of the furnace for the drying device can, in anyevent, be reduced. In addition thereto, the conventionally unused energyin the purified vapors can be fed into the drying circuit. For thisreason, the energy balance for operation of the overall installation isimproved. Depending on the operation conditions which depend, in turn,on the differences between sewage sludges, the operated installation canideally have a "positive" energy balance if one also takes theincineration of the dried briquette sewage sludge into consideration.

In a particularly preferred embodiment, a pipe contact drier is utilizedas the device for contact drying. Individual pipes are disposed within ahousing of the device for contact drying which are parallel and directedpreferentially in the axial direction of the device. The surfaces ofthese pipes are heated by a heating medium flowing within the pipes toproduce a proper and effective drying of the damp sewage sludge and tosimultaneously effect granulation of the dried product.

In an improvement, the device for contact drying comprises an upstreammixing device in which a dried sewage sludge consistency is produced.The mixing device facilitates mixing of a dried fine-grained granulatewith the e.g. mechanically pre-dried sewage sludge to form a pourablemixed product. Recycled dried sewage sludge and/or solid components canbe utilized as an additive granulate in the form produced by the vaporcleaning processes in the compressor utilized in accordance with theinvention. The addition of these products gives the sewage sludge whichis to be dried a peat-like, non-sticky structure to enhance and simplifythe drying process.

The method of drying in accordance with the invention effects a dryingof all sewage products occurring in community sewage processinginstallations and also those occurring in industrial installations whichis simpler than the conventional one. The process in accordance with theinvention can be utilized both in continuous as well as discontinuouslyoperating installations of various size.

Further advantages can be extracted from the description and theaccompanying drawing. The previous features mentioned and those to befurther described below can be utilized in accordance with the inventionindividually or collectively in arbitrary combination. The embodimentsmentioned are not to be considered exhaustive enumeration, rather haveexemplary character.

The invention is illustrated in the drawing and is more closelyexplained with reference to an embodiment.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a flow diagram of the method in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The method in accordance with the invention is partly shown in thefigure in a highly schematic fashion.

A flow diagram is shown which represents the development of the methodin accordance with the invention in a simplified fashion. Control andvalve devices are not shown, since same are known to one of averageskill in the art and are thereby utilized conventionally in anappropriate fashion.

A damp sewage sludge or a sludge of similar consistency is introduced inthe direction of arrow 9 into a first collection device (silo) 10. Thesewage sludge can be introduced into a mixing device 12 together with adry granulate located in a second collection device 11, wherein themixing device 12 is preferentially a horizontal solid mixer (aplough-share mixer). The peat-like mixing product then passes from themixing device 12 into a contact drying device 13 for thermal drying ofthe sewage sludge. The contact drying device 13 is preferentially acontact dryer having a plurality of internally disposed parallel pipesor surfaces providing large heat transfer. The peat-like mixing productcan be warmed through contact with the outer surfaces of the pipes sothat a dried final product and vapors result. For drying, a heatingmedium is initially heated inside a furnace 14 and can circulate withinthe contact drying device 13. The dried final product can be removedfrom the contact drying device 13 via an opening 15 and be furtherprocessed or used as fertilizer or fuel.

The vapors are passed into a compressor 16 in which they are brought toa higher pressure and therefore to a higher condensating temperature. Inaddition, the compression of the vapors causes a deposition of the dustcomponents contained therein. The compressed vapor is saturated, sincethe resulting over-heating can be reduced through additionalevaporation. The newly collected and deposited solid product can beintegrated into the processing cycle via the second collection device11. It can also be removed from the compressor 16 and utilized for otherpurposes or disposed of.

The purified hot water vapor passes out of the compressor 16 and intothe pipes of the contact drying device 13 to utilize the energy in thehot water vapor. This leads, in the stationary state, to reducedoperation of the furnace 14 so that, in the method for drying of sewagesludge in accordance with the invention, it may e.g. only be required tostart the processing cycle and to compensate for energy losses duringoperation of the installation. The furnace 14 is then only switched-induring subsequent processing when the amount of heat in the steamproduced in the compressor 16 is insufficient. The furnace 14 therebyonly initializes and supports drying the sewage sludge. The utilizationof hot purified and compressed vapors allows for a savings of 85-90% ofthe energy previously required for heating in conventional contactdrying processes.

In a method for the drying of sewage sludge or sludges having similarconsistency, a device (13) is utilized with which the vapors occurringduring contact drying are introduced to at least one compression stage.In this compression stage, the vapors are brought to a higher pressureand impurities are separated out of the vapors. The purified andpressurized vapor is introduced as a heating medium to the contactdrying device (13) for contact drying. The amount of energy utilized foroperation of the installation is thereby more efficiently applied to drythe sewage sludge. A substantially improved energy balance is thereforeachieved.

What is claimed is:
 1. A method for drying sewage sludge or sludges of a similar consistency, the method comprising the steps of:a) drying the sludge in a contact drying device; b) processing exhaust vapors generated during step a) in a compressor to produce heated gases and to separate out solids; and c) heating said contact drying device with said gases heated in step b).
 2. The method of claim 1, wherein said contact drying device is a pipe contact dryer.
 3. The method of claim 2, further comprising processing the sludge in a mixing device prior to step a) to change a consistency of the sludge.
 4. The method of claim 1, further comprising processing the sludge in a mixing device prior to step a) to change a consistency of the sludge.
 5. A method for drying sewage sludge or sludges of a similar consistency, the method comprising the steps of:a) drying the sludge in a contact drying device; b) directly feeding, without intervening filtering, exhaust vapors generated in step a) to a compressor; c) processing said exhaust vapors in said compressor to produce heated gases and to separate out solids; and d) heating said contact drying device with said gases heated in step c).
 6. A method for drying sewage sludge or sludges of a similar consistency comprising the steps of;a) drying the sludge in a contact drying device; b) directly feeding, without intervening filtering, substantially all exhaust vapors generated in step a) to a compressor; c) processing said exhaust vapors in said compressor to produce heated gases and to separate out solids; d) heating said contact drying device with said gases heated in step c); and e) removing said separated solids from said compressor. 